1. Field of the Invention
This invention relates to a character display device for displaying characters on a television screen.
2. Description of the Related Art
In recent years, in the United States of America, televisions having a so-called closed caption function have been put into commercial products for persons who have difficulty in hearing. This closed caption function is a function which displays a conversation, an announcement, or the like on television with a caption on a screen in real time, and when image information stored on record media, such as magnetic tape or magnetic disk, is reproduced, displays a conversation in the image information with a caption on the screen. That is, the function enables the user to select a caption display method as he or she desires. The system resembles multiplexed text broadcasting in Japan. Caption data for a caption in image information transmitted from a broadcasting station or reproduced from record media is carried on a predetermined horizontal scanning line (for example, the 21st horizontal scanning line) within a vertical blanking interval in the television signals. The caption data is decoded by the closed caption function, thereby displaying captions on the television screen.
The circuitry for providing the closed caption function is put into an IC which is contained in a television. The IC contains a video RAM (random access memory), character ROM (read-only memory), etc., for caption display. For example, the closed caption function provides caption display modes such as a text mode which displays 32-character.times.15-row information on a full television screen and a caption mode which can display 32-character.times.4-row information in a desired area of a television screen. When caption data in the text mode is decoded, character codes for 32 characters.times.15 rows are generated; when caption data in the caption mode is decoded, character codes for 32 characters.times.4 rows are generated. The video RAM mentioned above stores the character codes and consists of at least 480 (=32.times.15) addresses conforming to the text mode which requires a storage capacity larger than the caption mode. The 480 addresses of the video RAM correspond to character display locations of a maximum of 32 characters.times.15 rows displayed on the television screen on a one-to-one basis. Therefore, the video RAM is accessed in synchronization with horizontal and vertical synchronizing signals contained in television signals. The character ROM mentioned above stores data of character fonts (each consisting of horizontal m dots.times.vertical n dots) for predetermined characters that can be displayed on the television screen, and is accessed according to character codes in the video RAM. The corresponding character data is output from the character ROM. Since one row provides 32 character positions and one horizontal scanning line is one dot line, a dot pattern of horizontal m dots of character data is read for 32 characters in the horizontal direction and then repeated n times in sequence in the vertical direction.
As described above, caption data is carried within vertical blanking intervals of video signals, thus is updated at least in frame units. This means that a write and read of character codes into and from the video RAM are repeated in response to update of caption data. A microcomputer is used to write a character code into the video RAM and to read the character code for monitoring. For this reason, the write and read are executed in synchronization with a machine cycle in which the microcomputer performs operations. On the other hand, a read of a character code from the video RAM for on-screen display must correspond to each character display location as a matter of course, thus it is executed in synchronization with horizontal and vertical synchronizing signals.
An access when a character code is written into the video RAM and an access when the character code is read from the video RAM for display are executed asynchronously. Therefore, hitherto, a dual port RAM which can be accessed asynchronously for writing a character code and for reading it for display has been used as the video RAM.
However, if the video RAM is made up of dual ports, the number of input/output lines and the number of address decoders to access the video RAM increases, thus increasing the IC chip area and production costs. To suppress an increase in the chip area and a rise in cost, the character display area of one screen is divided into several portions and the video RAM can be made of a dual port RAM having the minimum display capacity which enables characters to be displayed on each division. However, this configuration requires that the storage contents of the video RAM should be changed each time the divisions are switched, and so program processing by the microcomputer becomes complicated.